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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14440
Experience:  Certified on Onan and Generac generators
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Onan B48g 20 hp not charging. Getting aprox 39 ac volts from

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Onan B48g 20 hp not charging. Getting aprox 39 ac volts from between two stator leads. When I check the dc voltage from middle plug on brand new from onan voltage reg. getting neg 24 dc volts. got same reading from two different reg onan pn(NNN) NNN-NNNNand(NNN) NNN-NNNN All rx1 seem good. This started last year when ammerter glass foged up and key switch wire to ammerter melted plastic plug a little. Thats when it stopped charging. all rx1 seems good from switch to meter and throgh fuse. Also replaced battery last year.

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Was this 24VDC reading with the battery wire attached, or with it off?


Customer: replied 5 years ago.
It was -24 vdc battery was attached and running aprox 3600 rpm

Disconnect the battery wire from the regulator and retest.


1. Put the black lead of your meter to ground, and the red lead to the battery pin of the regulator. How much voltage does it show with the battery disconnected?


2. Also test it with your meter set to ACV. Is there an ACV reading?


Customer: replied 5 years ago.

When you say battery disconnected do mean start it and unhook the cables from the battery or diconnect the orange lead from the regulator on the b+ pin? The -24vdc that I get is when the two black stator wires are connected and the orange b+ wire is disconnected from the regulator measured red lead at the disconnected pin and black lead to ground .

Yes, with the orange wire disconnected from the B+ pin.


Test for AC at the B+ pin, also.


Another thing I thought of, disconnect both AC leads from the regulator.

Set you meter to ohms, and place one lead to each AC wire. What is the resistance reading?

Now remove one lead, and place the lead to ground.

What is this resistance reading?


Customer: replied 5 years ago.

ok, engine running aprox 3600 rpm voltage at b+ pin dc=-24 volts ac=19 volts


motor not running ac lead to ac lead 0.1 ohm one ac lead to ground nothing

This is very weird.

The only problem I see here is the 19VAC at the B+ terminal of the rectifier. It should not be over 2 or 3 VAC.


As you are obviously aware, there are only 2 components in the charging system - the stator and the regulator.

The stator produces an AC output, approximately 40-60VAC @ 3600 rpm, which goes to the regulator.

The regulator is a set of diodes that convert the AC input to DC output, approximately 13.6 - 14.7 VDC @ 3600 rpm.


The fact that you are reading -24VDC makes me wonder 2 things.

I am certainly not trying to imply anything here - I am just keeping an open mind in attempting to help you get to the bottom of this.


1. Since you are getting a negative voltage reading, are your meter leads connected to the proper terminals of your meter?

2. With a consistent 24V reading, is it possible that your meter is set to the incorrect scale, or possibly bad? Have you calibrated/tested your meter against a known power source?



Customer: replied 5 years ago.

Meter is a sperry dsa500 check dc to a 9 volt battery out of the package 9.5 volts checked ac current to outlet 122.8 volts, terminals are correct. I actully was hoping it would be somthing easy that I was just missing. Maybe I should just replace the stator? Looks like a big job though. This engine is in a Jacobsen turff cat lawn mower that is from the 80's. I have replaced the regulator in the past and then it charged again.

Actually, with the tests you have run, the stator is doing what it should.

By the book, the regulator is bad.


This is why it is so confusing - 2 different regulators should not be doing the same thing.

Changing the stator is not that difficult. The hardest part is getting the flywheel off.


The(NNN) NNN-NNNNis the correct regulator for this engine, so having a wrong part is not an issue.


You could try a new stator, but personally, I would go with a different regulator.


After that, I'm afraid I will be of no further help - we have covered the entire system.


Hank F. and 6 other Small Engine Specialists are ready to help you
Customer: replied 5 years ago.

Well a new regulator is $60.00 not going to buy another one from onan. Have a new one from last summer and a new from Monday so I will have to take it in somewhere. Changing the stater probally isnt a big deal its getting at it, I think the engine will have to come out to get at it. Thanks for your help

I can't say if you will need to pull the engine or not, but it is a possibility.

It just depends on how the engine is mounted.

The flywheel has to come off, as the stator is mounted behind the flywheel.

The stator itself just has 3 or 4 small bolts holding it in place.


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